This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations employing alkaline agents in conjunction with sulfonate surfactants.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. To date one of the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates within a designated equivalent weight range and under controlled conditions of salinity. For example, in a paper by W.R. Foster entitled "A Low-Tension Waterflooding Process", JOURNAL OF PETROLEUM TECHNOLOGY, Vol. 25, Feb. 1973, pp. 205-210, there is disclosed a procedure which involves the sequential injection of a protective slug, a surfactant slug, and a mobility control slug. The protective slug is an aqueous solution of sodium chloride which is injected in order to displace the reservoir water ahead of the subsequently injected surfactant slug. This slug is substantially free of divalent ions which would tend to precipitate the subsequently injected surfactant.
The surfactant slug comprises an aqueous solution of petroleum sulfonates and contains sodium chloride in a concentration, typically about 1.0 to 2.0 weight percent, which will promote the desired low interfacial tension between the injected water and the reservoir oil. The subsequently injected thickened water slug contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide an initial viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. Thereafter a driving fluid such as produced field brine is injected in order to carry the process to conclusion.
Another waterflooding procedure, in which surfactants are formed in situ, involves alkaline waterflooding. In this process an aqueous solution of an alkali metal or ammonium hydroxide or carbonate is injected in order to neutralize organic acids in the reservoir oil to produce the corresponding alkali metal or ammonium salts. Alkaline waterflooding has been proposed in regard to various recovery mechanisms, i.e., to lower the interfacial tension between the reservoir oil and the injected water, to alter or even reverse the wettability of the reservoir, e.g., from oil-wet to water-wet, or for the purpose of mobility control by the formation of a relatively viscous oil and water emulsion.
An improved alkaline waterflooding process is described in U.S. Pat. No. 3,927,716 to Ralph F. Burdyn, Harry L. Chang, and William R. Foster, and entitled "ALKALINE WATERFLOODING PROCESS". In this process an aqueous alkaline solution is employed in which the alkalinity and monovalent salt salinity of the solution are controlled within defined ranges in order to result in low oil-water interfacial tensions which enhance the microscopic displacement of oil from the interstices of the reservoir rock. A thickened water slug may be used for the purpose of mobility control in the alkaline waterflood. As recognized in the Burdyn et al. patent, the results achieved by alkaline waterflooding and the roles played by the various recovery mechanisms involved depend to some extent upon the molecular weight distribution of the organic acids within the reservoir oil.
An additional factor which is relevant to the efficacy of an alkaline waterflood is the total acid content of the reservoir oil. This is commonly measured by the "acid number" which is defined as the milligrams of potassium hydroxide required to neutralize the acids in one gram of crude oil in a nonaqueous type titration. Various procedures have been proposed for the alkaline waterflooding of those reservoirs in which the crude oil is considered to have an inadequate acid content as indicated by the acid number. One technique disclosed in U.S. Pat. No. 3,195,629 to Leach involves the injection of air, peroxides, or other oxidizing agents into the reservoir in order to oxidize the oil in situ to form additional organic acids. Additionally, a preoxidized oil bank may be injected or high molecular weight acids may be added to the injected oil bank. In any case, an aqueous alkaline solution is injected in order to form the sodium salts of the organic acids. Yet a further technique disclosed in U.S. Pat. No. 3,344,858 to Gilchrist et al. involves an alkaline waterflood process in which an aqueous alkaline slug is injected first and then followed with a hydrocarbon slug containing organic acids. These acids are neutralized in situ to form the corresponding surface-active salts.
A further example of surfactant waterflooding is found in U.S. Pat. No. 3,804,171 to Krehbiel et al. which discloses the use of an anionic waterflood additive characterized as an overbased sulfonate. The overbased sulfonate is arrived at by adding a base component, such as an alkali metal hydroxide, carbonate or bicarbonate, to a sulfonate derived from a pale oil extract. The base component is added in an amount over that required to neutralize the sulfonte such that the ratio of the weight of excess base component to the weight of the sulfonate is within the range of 0.03 to 2.0 and desirably within the range of 0.20 to 1.0. Overbasing may be arrived at in situ or prior to injection. In the former case, the sulfonate derived from a pale oil extract may be injected into the formation and this sulfonate slug then followed by an effective amount of an alkali metal hydroxide or alkali metal carbonate solution.
A technique similar to that described above is disclosed in U.S. Pat. No. 3,804,170 to Krehbiel et al. In this patent, the overbased sulfonate is derived from heavy coker gas oil and in addition a sufficient amount of brine may be added to the slug of the anionic waterflood additive to provide a brine level in the reservoir of about 0.5 to 10 percent salt. Yet a further disclosure of employing an overbased anionic waterflood additive is found in U.S. Pat. No. 3,847,823 to Clark et al. In this case, the overbased component comprises a mixture of water-soluble sulfonate having an average equivalent weight of about 200 to about 400 and an oil-soluble sulfonate having an equivalent weight of about 400 to about 600. The sulfonates in the mixture are generally present in a ratio such that the average equivalent weight of the mixture is about 350 to 500, and desirably about 375 to about 475. The neutralized sulfonate mixture may be present in an amount within the range of 0.5 to 25 weight percent and may contain a chelating agent such as trisodium phosphate monohydrate to chelate calcium and/or magnesium ions.